4.4.2: Coastal impact of tide and classification
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The semi-diurnal or diurnal rise and fall of the water level creates a so-called intertidal zone that is exposed during low water, and submerged during high water. In the absence of waves (or with only very low-wave energy) tide-dominated coasts develop wide low-gradient tidal flats (see for example Fig. 2.11) in the intertidal zone and subtidal zone (the area only infrequently exposed, during extreme low tides). Tidal currents in combination with the horizontal translation of the water line determine the morphology of these tidal flats. Due to the low-energy conditions tide-dominated coasts generally consist of relatively fine sediments. The sediment distribution patterns are exactly opposite to those on wave-dominated coasts: since tidal currents increase in strength for larger water depths, the finest sediments occur on the often muddy flats and in the wetlands of the upper intertidal zone and the coarser sandy sediments occur in the lower intertidal zone and further seawards. From the upper part of the intertidal zone to the supra-tidal zone (only submerged with spring tides or storm surges) salt marshes are well developed. In tropical to sub-tropical regions, mangroves occupy the intertidal zone.
Tide-dominated flats occur for large tidal ranges and small wave heights. This is the case for macro-tidal coasts, but tide-dominated coasts can also be found for micro-tidal regimes as long as the wave-energy is very low (for instance in an estuary). It is rather the relative importance of tide and waves than the absolute tidal range that determines the coastal character. Hayes (1979) and Davis Jr. and Hayes (1984) distinguish five classes, based on a combination of the tidal range and wave energy classification (Fig. 4.13).
A useful parameter to delineate between wave and tide influence is the relative tidal range \(RTR\) as introduced by Masselink and Short (1993):
\[RTR = MSTR/H_b\]
where \(MSTR\) is the mean spring tidal range and \(H_b\) is the wave height just before breaking. For \(RTR < 3\) we find the wave-dominated beaches as described in Sect. 4.4.2. For \(RTR > 15\) the beaches gradually approach the pure tidal flat situation. For the intermediate range, we find beaches shaped by waves with some distinct tidal characteristics, such as the wide intertidal zone. The effect of the tide is that the zone of wave attack shifts with the tidal phase. This means that there is not enough time for wave-dominated bar morphology to develop. In other words: tides smear beach morphology. Tides develop wide, low amplitude parallel or sub-parallel bars (tidal ridges) on the intertidal beach, especially for macro-tidal regimes. During storms, waves flat- ten these ridges out. Tide-dominated coastal features are treated more extensively in Ch. 9.